In many wireless communication systems, a frame structure is used for data transmission between wireless stations such as a transmitter and a receiver. For example, the IEEE 802.11 standard uses frame aggregation in a Media Access Control (MAC) layer and a physical (PHY) layer. In a typical transmitter, a MAC layer attaches a MAC header to a MAC Service Data Unit (MSDU), in order to construct a MAC Protocol Data Unit (MPDU). The MAC header includes information such as source addresses (SA) and a destination address (DA). The MPDU is a part of a PHY Service Data Unit (PSDU) and is transferred to a PHY layer in the transmitter to attach a PHY header thereto to construct a PHY Protocol Data Unit (PPDU) The PHY header includes parameters for determining a transmission scheme including a coding/modulation scheme. Before transmission as a packet from a transmitter to a receiver, a preamble is attached to the PPDU, wherein the preamble can include channel estimation and synchronization information.
According to the IEEE 802.11e specification, “Amendment: Medium Access Control (MAC) Quality of Service (QoS) Enhancements”, (incorporated herein by reference), the QoS enabled stations (QSTAs) must maintain a transmit MSDU timer for each MSDU passed to the MAC layer. MSDU delivery ensures that the information in each MSDU is delivered between the MAC service access points.
A Management Information Base (MIB) attribute “dot11EDCATableMSDULifetime” in the MAC layer specifies the maximum amount of time allowed to transmit a MSDU for a given access category (AC). A transmit MSDU timer is started when the MSDU is passed to the MAC layer. If the value of this timer exceeds the appropriate entry in the “dot11EDCATableMSDULifetime” attribute, then the MSDU, or any remaining, undelivered fragments of that MSDU, are discarded by the source QSTA without any further attempt to complete delivery of that MSDU. Since the existing Automatic Repeat Request (ARQ) scheme simply relies on the “dot11EDCATableMSDULifetime” attribute, it is possible that the MSDU is discarded after a few retries. Discarding a MSDU can negatively impact the QoS for some applications such as video.
The QSTA in a wireless station obtains a transmission opportunity (TXOP), and forms a MSDU for transmission. If the current TXOP duration expires before the MSDU is successfully transmitted, then the QSTA re-contends the communication medium (channel) because the QSTA cannot utilize the channel longer than the TXOP duration. The overhead of successfully obtaining another TXOP is significant, and is typically the sum of the following periods: Inter-Frame Space (xIFS), backoff after a collision on a shared channel, Request To Send (RTS), and Clear To Send (CTS) control frames. As a result, the MSDU can expire before it is scheduled for retransmission.
Moreover, the IEEE 802.11 MAC protocol is contention based, and as such, it is difficult to predict when the QSTA will re-obtain the channel for transmission. FIG. 1 shows an example timing diagram scenario 10 wherein within a TXOP 20, a MSDU 30 is not successfully transmitted due to collision. As a result, since the QSTA cannot utilize the channel longer than the TXOP duration, the QSTA must re-contend the channel for another TXOP period.
FIG. 2 illustrates a timing diagram scenario 40 wherein the QSTA contends for the channel to obtain another TXOP 41 so that the MSDU blocks (packets) which could not be transmitted successfully in the previous TXOP 20 can be transmitted. A time period Tdelay 42 represents the delay from the last TXOP 20 until a remaining MSDU 30 can be sent. The Tdelay 42 depends on a time periods Tbusy 44, and a backoff interval 45 may be quite large. In this scenario, the remaining MSDUs expire before their transmission can be attempted again. After the QSTA re-obtains the channel, because the remaining MSDUs' transmit timers exceed the “dot11EDCATableMSDULifetime” attribute during the Tdelay period 42, the expired MSDUs are discarded. The existing ARQ scheme in the IEEE 802.11 standard does not provide further attempts at transmission of expired MSDUs in order to preserve and/or improve the quality of QoS for time-sensitive applications, negatively impacting QoS sensitive applications.